Metal containers are commonly used for food and beverage packaging. The containers are typically made of steel or aluminum. A prolonged contact between the metal and the filled product can lead to corrosion of the container. To prevent direct contact between filled product and metal, a coating is typically applied to the interior of the food and beverage cans. In order to be effective, such a coating must have adequate properties that are needed for protecting the packaged products, such as adhesion, corrosion resistance, chemical resistance, flexibility, stain resistance, and hydrolytic stability. Moreover, the coating must be able to withstand processing conditions during can fabrication and food sterilization. Coatings based on a combination of epoxy and phenolic resins are known to be able to provide a good balance of the required properties and are most widely used. There are industry sectors moving away from food contact polymers made with bisphenol A (BPA) and bisphenol A diglycidyl ether (BADGE), which are the building blocks of the epoxy resins. Thus, there exists a desire for the replacement of epoxy resin used in interior can coatings.
Polyester has been of particular interest to the coating industry to be used as a replacement for epoxy resin because of its comparable properties such as flexibility and adhesion. It is known by one skilled in the art that crosslinking between common polyester and phenolic resin is too poor to provide adequate properties for use in interior can coatings. Specifically, conventional polyesters having hydroxyl functionalities are not reactive enough with phenolic resins under curing conditions to provide adequate cross-linking density, resulting a coating that lacks good solvent resistance.
There remains a need for a coating system based on curable polyester resins and phenolic resins that is capable of providing effective crosslinking to achieve the desirable coating properties.